A well known problem area in the design of power supplies and specifically in the area of interrupted power supplies, is that of connecting a low-level logic signal to a high voltage switch. Logic circuits produce low-level logic pulses referenced to ground of the logic circuit. Power supply devices have one or more switches which are to be operated by the logic signals, but which operate at high voltage, e.g., hundreds of volts from ground. A high voltage IGBT, MOSFET, etc. may typically require a 15 volt signal on its gate to turn it on, and 0 volt signal referenced to its own ground to turn it off. Accordingly, the output from the logic circuitry cannot simply be coupled straight through to the high voltage switching circuitry. What is needed is a gate drive which not only transforms the signal to the proper voltage differential and ground reference, but which provides isolation between the logic circuitry and the switching circuitry.
In order to deal with the above problem, the prior art discloses the use of various isolating elements, including opto-isolators and transformers. Opto-isolators, however, are known to introduce reliability problems. Transformers are difficult to miniaturize, and consequently are a significant cost item, and take up space. Although there has been a great deal of activity in miniaturizing transformers, it still remains a substantial problem to provide a low-cost transformer along with a low-cost drive circuit for driving the transformer primary so as to result in efficient coupling of the logic signal through to the transformer secondary. As examples of the prior art, reference is made to U.S. Pat. No. 3,760,198, Mori et at., which describes a standard transformer circuit producing sharp edge pulse transmission through the transformer, with circuitry for reshaping the pulses that are distorted in transmission through the transformer. Reference is also made to U.S. Pat. No. 4,433,719, Planer et at., showing standard pulse transformer techniques. In the area of transformer design, and particularly design of low-cost transformers suitable for gate drive circuits, reference is made to U.S. Pat. No. 4,342,143 to Jennings; U.S. Pat. No. 4,785,345 to Rawls et at.; U.S. Pat. No. 4,803,453, Tomono et al.; and U.S. Pat. No. 4,959,630, Yerman et at.
FIG. 1 shows a prior art gate drive circuit disclosed in pending U.S. application Ser. No. 939,311, now U.S. Pat. No. 5,399,913, assigned to the same assignee as this invention. As illustrated in FIG. 1, the two clock signal inputs provide a sharp edged carrier which is inputted to the input of transformer T1. The ENABLE signal, which is the logic signal to be transmitted through to the gate or other input node of the high voltage switching device, is inputted into the primary circuit so as to modulate the carrier. The modulated carder passes through the transformer to the secondary, and the circuitry between the transformer secondary and the output terminals at the gate drive (indicated as gate and emitter) performs the function of demodulating the waveform, shaping the signal, and deriving power from the signal passed through the transformer for switching the switching device. In this prior art circuit, each of the two transistors typically has to handle about 600 ma. Most of this current is magnetizing current required in order to produce a signal through the transformer.
In FIG. 1, transformer T1 is typically a conventional ferrite core, torroidal transformer. The transformer has few turns and a small core to keep the cost low. However, since this transformer has limited volt-second capability, the drive components must operate at high frequencies, e.g., 2 MHz. This type of construction tends to have significant magnetization current, requiring the components to switch at both high frequency and high current. Here, the primary drive components switch the transformer in a push-pull configuration, which causes the edges of the waveform to be sharp, or square. The emission spectrum of this type of circuit can cause unwanted frequencies in the hundreds of MHz range. In addition, the primary drive components must supply the load current and switch the substantial magnetizing current, which causes stress and tends to reduce overall reliability.
In order to provide a less expensive transformer, such as the type used in this invention, an air-core-type of transformer can be used. However, taking the core out of the transformer generally requires an even higher frequency, which in turn generally leads to a higher magnetization current requirement. Thus, the potential reduction of the expense involved in the transformer T1 by using a lower cost air-core transformer would be expected to result in even greater demands upon the drive circuitry for driving the transformer primary, which in turn would result in costs offsetting the cheaper transformer costs. Accordingly, what is required is an improved input or primary drive for a gate drive circuit, which solves the above problems and results in reduced cost. Such reduced cost is important in an apparatus such as an uninterrupted power supply (UPS), which may typically require three or four such gate drive circuits to couple low-level logic signals to high voltage switching devices.